Displacement reactions as redox reactions
3 marks · NormalExplain, in terms of electron transfer, why iodine does not displace bromine from a solution of potassium bromide. Include references to oxidation and reduction in your answer.
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Iodine cannot oxidise bromide ions because it is less reactive; it is unable to gain electrons from bromide ions. Therefore, no redox occurs: bromide ions are not oxidised, and iodine is not reduced. Iodine is not a strong enough oxidising agent to displace bromine.
Must show: Reference to electron transfer, relative reactivity, inability to oxidise, use of oxidation/reduction terminology
Common mistakes: Confusing direction of displacement; omitting mention of electron transfer
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Marking points
M1 · 1 mark · States iodine is less reactive than bromine or is not a strong enough oxidising agent.
M2 · 1 mark · Explains bromide ions are not oxidised, and/or iodine is not reduced (relates to electron transfer).
A1 · 1 mark · Links answer explicitly to the lack of electron transfer (no oxidation/reduction occurs).
Worked solution
Iodine is less reactive than bromine and is not a strong enough oxidising agent to remove electrons from bromide ions. As a result, no displacement occurs because bromide ions cannot be oxidised by iodine (no electron is lost by Br−), and so iodine is not reduced (does not gain electrons). Therefore, no redox reaction takes place.